In internal combustion engines and particularly in automotive engines, it has become popular to perform feedback control of the air/fuel mixing ratio by utilizing an air/fuel ratio detector such as an oxygen sensor installed in an exhaust passage as a means for providing a feedback signal. For an engine equipped with a carburetor, it is usual to provide an electrically operated flow control valve means to either a fuel passage or an auxiliary air-admitting passage connected to a fuel passage and control the operation of this valve means by a control signal produced on the basis of an actual air/fuel ratio value detected by the sensor.
As the electrically operated flow control valve means, it is preferred to employ a solenoid valve of the on-off functioning type mainly because of its better responsiveness compared with other practicable methods such as the use of a needle valve operated by a linear solenoid or a stepping motor through a gear train. Where the air/fuel ratio control system utilizing a solenoid valve of the on-off functioning type is designed so as to control two fluid passages as in the case of controlling not only the rate of fuel feed through a main fuel discharge passage in a carburetor but also the rate of fuel feed through a slow fuel discharge passage of the same carburetor, it is indispensable to provide one solenoid valve for each fuel discharge passage, so that the control system must include two solenoid valves.